A bomb blast hit a bus in Cairo's Nasr City district, injuring four people, according to a spokesman for Egypt's interior ministry.

The homemade device had been put on a traffic island and a second one, found nearby, was dismantled.

Attacks across Egypt over the last few days have raised fears of intensifying violence in the run-up to a constitutional referendum in January.

The interim government is pushing ahead with an army-backed plan for political transition.

The government has vowed to fight "black terrorism" after Tuesday's deadly attack on a police compound in the city of Mansoura which also injured about 140 people.

Bomb attacks and shootings have become an almost daily occurrence in the Sinai peninsula since the army toppled Islamist President Mohammed Mursi in July following mass protests against his rule.